The present invention relates generally to probes and, more particularly, to probes for electrical testing of electronic packages and similar devices.
Electrical testing of electronic packages and similar devices often requires that temporary mechanical and electrical contact be made between test probes, which are connected to the electronics contained within a test system, and metallic features on the surface of the device under test. Examples of some types of electrical testing include tests for opens and shorts of bare substrates and printed circuit cards, and functional tests of populated electronics packages.
Among the metallic features of electronic packages requiring testing are the input/output or xe2x80x9cI/Oxe2x80x9d pads, generally located on the BSM (bottom side metal) side of a package, such as a ceramic substrate. One type of test probe fixture or head used to test such metallic features is a xe2x80x9cbed-of-nailsxe2x80x9d or xe2x80x9cneedle card.xe2x80x9d This test head is generally a grouping of individual, spring-loaded test probes commonly known as xe2x80x9cpogo pins.xe2x80x9d A single pogo pin is positioned in the bed-of-nails or needle-card test fixture for each I/O pad or other metallic feature on the electronic package to be tested. The tip of one pogo pin contacts a corresponding, individual I/O pad to test it. Each pogo pin is a single-beam test probe comprised of a xe2x80x9cpistonxe2x80x9d which slides in and out of a xe2x80x9ccylinder.xe2x80x9d A spring inside of the cylinder exerts a force against the piston, and the outward force of the compressed spring biases the tip of the piston into contact with the I/O pad during the test operation.
Non-conductive contamination on the surface of metallic features under test may prevent effective and accurate testing by the bed-of-nails or needle-card type fixture described above. For example, such contamination can prevent the test probe from making the necessary, low resistance, electrical connection to the metallic feature, thus creating invalid or false test results. Such contamination can manifest itself, for example, as widely scattered pieces of dielectric material which are on the surface of the metallic features.
The testing apparatus, probes, and methods of the current art are often ill equipped to take account of such contamination. Generally, if such contamination is present in a location such that it prevents an electrical connection between the test probe and the metallic feature to be tested, the electronic package is deemed functionally defective. In the case of testing bare ceramic substrates for opens and shorts, if the test system is unable to make the required electrical connection with the area under test, such failure is interpreted, incorrectly, as an undesirable, open circuit. If time and costs allow, the part may be re-tested to verify the validity of such finding, or the substrate may be discarded as defective. In either event, additional time or money is expended as a result of the drawbacks of the current testing methods and apparatus.
It should be noted that difficulties in accurate and efficient electrical testing can arise even when the dielectric contamination is quite small relative to the size of the metallic feature and the tip of the test probe. It is sufficient for the dielectric contaminant to prevent good contact between the tip of the test probe and the metallic feature for the tester to detect, incorrectly, that the metallic feature is flawed with an open. Contaminants with diameters measured in microns have been seen to cause such false opens.
Prior art attempts to address the above-described difficulties have generally not been satisfactory. For example, pogo pin geometry has been varied at the distal, test ends of pogo pins in an attempt to improve electrical contact. The test ends have been equipped with multiple points, such as the so-called xe2x80x9ccrown-headxe2x80x9d pogo pin. The multiple tips at the end of such pogo pins are rigidly connected to each other and are mounted to the same, single spring; therefore, the multiple tips or points move together as a unit. As such, a piece of contaminant encountered by just one of the tips often prevents all tips of the xe2x80x9ccrownxe2x80x9d from making the desired electrical connection.
Certain test probes make use of brush tips, generally comprising multiple, closely packed, pliant filaments. Examples of this approach are found in U.S. Pat. No. 5,397,996, titled xe2x80x9cContinuity Tester Using A Brush Topped Probe,xe2x80x9d and in IBM Technical Disclosure Bulletin, Vol. 12, No. 5, dated October 1969. The single continuity tester uses many strands of wire in a brush to make relatively imprecise contact with the device under test. Accordingly, it is ill suited for testing more closely spaced conductive features or for other test operations requiring the probe to make precisely positioned contact with features under test.
There is thus a need for a test probe which functions more reliably in the presence of non-conductive contaminants on the electrical surface to be tested. There is a further need for such a test probe to be suitable for the ever-shrinking geometries of the electronic packages of today and tomorrow.
To meet these and other needs, and in view of its purposes, the present invention, according to one of its aspects, provides a multi-point test probe used in a test system for testing a feature of a device. The probe has a test head moveable relative to the device, and a cluster of at least two buckling beams secured to the test head. There may also be more than two buckling beams secured to each cluster of the test head. The buckling beams extend generally outwardly from the test head and terminate in distal ends. The buckling beams have strength characteristics selected so that they establish sufficient electrical contact for testing the device when the test head is moved into the appropriate position in relation to the device. The buckling beams deflect laterally when subjected to an axially compressive force. The distal ends of the buckling beams are spaced from each other by a distance no greater than the maximum linear dimension of the feature being tested. The probe has suitable circuitry to electrically connect each of the buckling beams in parallel to the test system. This physical and electrical arrangement of the buckling beam cluster allows it to make electrical connections to the features under test even if only one of the buckling beams contacts the features.
In accordance with another aspect of the present invention, there are either three or four of the buckling beams, and they extend from spaced locations on mounts on the test head. The beams are spaced in radial increments of 120 degrees in the case of three buckling beams, and 90 degrees in the case of four buckling beams. In accordance with yet another aspect of the present invention, there are multiple clusters of the buckling beams discussed above; the clusters are arranged in a matrix corresponding to an array of features on the device under test. According to still another aspect of the present invention, the beams have substantially the same operative length to define a test plane for the probe coinciding with the distal ends of the beams. The distal ends of the buckling beams which encounter a contaminant during testing are deflected out of this test plane as the test head moves into its test position.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention.